The faint young sun climatic paradox: Influence of the continental configuration and of the seasonal cycle on the climatic stability

A quasi-three-dimensional climate model is used to study the early state of the Earth when the solar luminosity was 70% of the present value, Usually, climatic simulations going back to this period lead to a completely frozen planet contrasting with the geologic evidences of sedimentary rock formation and thus of the presence of liquid water at the surface of the continents during the Archean (4.6-2.5 billion years before present). Here, several model simulations are performed for solar luminosities varying between 0.7 and 1 times the present value. Using the present-day continental configuration and taking the seasonal cycle into account, a steady state is found in which glaciation is complete but snow covers only some oceanic coasts, leaving the continents essentially snow-free. As a result, the albedo of the continental area is strongly reduced compared to that of the frozen ocean, Some continental temperatures can almost reach the freezing point of water in summer (-1 degrees C in the center of Eurasia). This result can be explained by the behavior of the detailed hydrologic cycle included in the model. During the decrease of the solar luminosity, the jump to a completely frozen Earth occurs when the solar luminosity reaches 0.86 times its present value. The behavior of the climatic system is substantially different with a global ocean configuration. In the absence of land surfaces, the meridional heat transport, explicitly calculated, is less effective and the glaciation of a model latitude zone does not lead to the glaciation of its equatorward neighbor. The climate instability is relatively local and the jump to the completely frozen state is much more progressive than in the case of the modem continental configuration. The role of the seasonal cycle in the paleoclimatic simulation is also studied. Due to the non-linearity of the model, removing the seasonal cycle drives the system to an increase of the annual mean planetary albedo and to a decrease of the relative value (0.82) of the critical solar luminosity at which the jump to the completely frozen solution occurs.